The present invention relates to an indicator terminal for use in devices for measuring the thickness of metallic coatings on metals and more particularly to a novel indicator terminal for a coulometric measuring device.
Coulometric measuring devices such as those disclosed in U.S. Ser. No. 273,085, filed June 12, 1981 now abandoned and Ser. No. 335,095 filed Dec. 29, 1981 generally use an electrolytic solution to dissolve a predetermined minute area of coated materials, as in a determination of the quality of anti-corrosion protection. Based on known elecrrolytic parameters which can be measured during such dissolution, the coating thickness is ascertained to a very high degree of accuracy. Since the dissolution of coated material is a form of destructive testing, such measuring devices are usually designed to operate over a substantially small circular area with a diameter that can range from as low as 3 mm down to 1 mm, thereby mimimizing the affected surface portion.
The coulometric measuring device usually isolates the area to be measured by temporarily surrounding such area with a leak-tight collar, cuff or terminal that prevents leakage of electrolytic solution. A nozzle for delivering the electrolytic solution to the surrounded area is generally recessed to provide a working space in which the electrolytic solution can flow over the surrounded area. In the aforementioned U.S. applications, an annular clearance space around the nozzle is provided to receive flowage for pulsation of the electrolytic fluid that has entered the working space so that an effective exchange of fluid can be made.
In a hitherto known embodiment of the microindicators, a rubber terminal is used, shaped as a cylinder provided with a center hole, having a diameter of 1 mm. In the center of the terminal, usually in a distance 0.5 mm from the measured surface, an outlet of a nozzle is situated, through which the working solution passes to the surface. The outer diameter of the nozzle limits the remaining space (i.e.: annulus) between the nozzle and the inner hole of the terminal. Through this space the entire volume of the working solution passes at its continuous or pulsed change during the measuring operation. The minaturization of this element of the indicator causes undefined changes of the working solution that results in dispersion variances in the measured results. The needed pressure for delivery of the working solution to the surface and the minimum space for suction of solution together with the electrochemical effect of an anode dissolving of the coating-hydrogen on the cathode--causes the appearance of bubbles in the space of the terminal which worsen the exchange of the working solution and cause the electrolyte to flow in undefined turbulence. The known dimensions of the nozzle as well as the electrically unconductive materials used cause a misalignment of the nozzle resulting from manipulation and inherent lack of strength of the material. These result in an eccentric flow of solution and in the creation of screened spots where the etching of the coating is slowed. The etched surface thus will not correspond to the area of the rubber terminal.
Because the working space in which the electrolytic fluid flows is of extremely small volume, any distortions of the leak-tight sleeve, cuff, collar or terminal will affect the volume of the working space, thereby having an adverse effect on the accuracy of the coulometric measurement. Furthermore, during the electrolytic action, the fluid flowing over the surrounded surface may develop bubble formations that interfere with the fluid flow and often cause turbulence or erractic flow patterns. Therefore, non-uniform depletion of the coating in the surrounded area can occur which diminishes the accuracy of the coulometric measurement. Consequently, it is difficult to obtain measurements that can be repeated when a series of such measurements are taken.
It is thus desirable to provide a terminal for a coulometric measuring device that does not deform in providing a leak-tight seal, and which promotes uniform laminar flow of the fluid in the working space.
Among the several objects of the invention may be noted the provision of a novel indicator terminal for a coulometric measuring device, which maintains volumetic consistency of the working space and a novel indicator terminal that has provision for taking up the flow of electrolytic fluid in a substantially laminar flow pattern to draw away any bubbles that may be formed in the working space.
Other objects and features will be apparent from the following disclosure.